- Email: [email protected]
Electrochemical oxidation of graphite in an aqueous medium: intercalation of FeCl4−
Carbon. Vol. 31, No. I. pp. 223-226, Printed in Great Britain.
OCOX-6223/93 $6.00 + .@I Copyright 0 1993 Pergamon Press Ltd.
1993
ELECTROCHEMICAL OXIDATION OF GRAPHITE IN AN AQUEOUS MEDIUM: INTERCALATION OF FeCl; H. SHIOYAMA, M. CRESPIN, A. SERON, R. SETTON, D. BONNIN, and F. BEGUIN C.R.M.D., C.N.R.S., I b, rue de la F&ollerie, 4507 I Orl&ans, France (Received 6 May 1992; accepted in revisedform 24 July 1992) Abstract-Graphite was oxidized electrochemically in an aqueous solution of 2.5 moles of FeCl,, I .75 moles of HCI and 6 moles of HzO, which leads to the formation of stage n iron chloride-graphite intercalation compounds (GICs) (where n > 2). The interplanar distance d, of the GIC is -9.4 A; this value is similar to that of the FeClj-GIC obtained by the vapour phase method. Miissbauer spectroscopy shows that the FeC14 ion exists in the interlayer spacing of graphite. The existence of co-intercalated species is suggested by the results of elemental analysis, thermogravimetry and so on. Key Words-Electrochemical
intercalation, water, ferric chloride, hydrochloric acid.
1. INTRODUCHON
2. EXPERIMENTAL
Although Graphite intercalation compounds (GICs) with metal chlorides are usually prepared by the twozone method[ I], the preparation of the GIC from a liquid phase has also been studied by many researchers. Molten metal chloride systems were used successfully to synthesize corresponding GICs[2,3]. Metal chlorides dissolved in organic solvents were also used for GIC syntheses, with the oxidant necessary to drive the reaction provided by gaseous chlorine bubbled through the solution[4], formed by a photochemical reaction[S], or created by an electric current[6,7]. In these cases, solvents were always cointercalated, giving GICs with properties different from those ofthe corresponding binary GIC obtained by vapour phase intercalation. Compared with other preparation methods, the electrochemical preparation of GICs has several advantages. For example, the stage of the sample can be correlated with the applied potential or the charge passed during the electrochemical intercalation, hence this process is applicable not only to the analysis of the formation process of GIC but also to the well-controlled synthesis of GIC. Water has never been used as an electrolytic solvent for the electrochemical intercalation of metal halides because of its limited potential window. In 1990, Futamata[8] gave a clue to the electrochemical intercalation of ZnBr, into carbon plastic electrodes by repeating charge-discharge cycles in aqueous solutions, but intercalation into ordinary graphite using a conventional electrochemical technique was unsuccessful. In this paper, we present the electrochemical preparation of FeCl;-GIC in an aqueous medium. Some properties of the product are also described.
The composition of the solution used was 2.5 moles of FeCl,, 1.75 moles of HCl, and 6 moles of H,O. Highly oriented pyrolytic graphite (HOPG) from Union Carbide was used as the host. A slab of HOPG (ca. 10 mg) was used for the working electrode; electrical contact was made by a bundle of nongraphitized ex-PAN carbon fiber. A glassy carbon rod was used for the counter electrode and the saturated calomel electrode (SCE) was adopted as a reference electrode, to which all potential values will be referred hereafter. Cyclic voltammetry was carried out with the help of a triangle wave signal generator (Tacussel, type GSTP4) and a potentiostat (Tacussel, type PRT 1O0,5). Galvanostatic experiments were performed using a DC current generator. The relation between the potential of the working electrode and electric quantity transferred in the cell was continuously measured with a high impedance X/t-recorder. The electrochemical treatments could be stopped at any desired time to obtain the X-ray diffraction pattern of the sample. During this measurement, the sample was covered with thin polyethylene film (0.02-mm thick) to prevent decomposition by moisture. After washing with a large amount of 6-N HCl solution to remove any adhering iron chloride, the samples were transferred into ampoules, sealed under vacuum, and sent to the Service Central de Microanalyse du CNRS (Vernaison, France) for elemental analyses. MGssbauer spectra were recorded using a Lescint AME Mijssbauer driver with a lo-mCi 57Cosource in rhodium matrix at room temperature. As references (CH,),N+FeCl; crystal and FeCl,-GIC obtained by vapour phase synthesis were used. 3. RESULTS AND DISCUSSION
Please send correspondence concerning this paper to Dr. H. Shioyama, Government Industrial Res. Inst., Osaka, Midorigaoka I-8-31, Ikeda, Osaka 563, Japan.
223
One of the major problems of water as a solvent for electrochemical reaction is its limited potential
224
H.
1 (mA) 1.04
1(mA)
f
(b)
1.0
eta1
SHIOYAMA
I
of such a concentrated solution of hydrochloric acid is that FeCl; is the average and only important solute formed from FeClJ9]. Hence, it was expected that the FeCld ion could be intercalated during the anodic oxidation of graphite in this solution. A typical cyclic voltammogram obtained during the initial potential sweep is shown in Fig. l(a). Compared with a voltammogram [Fig. l(b)] of nongraphitized ex-PAN carbon fiber (i.e., the voltammogram of the working electrode without HOPG), anodic peaks are observed. In order to interpret the voltammogram, the potential sweep was stopped at several points (a to h) indicated in Fig. l(a) to obtain X-ray diffraction patterns ofthe sample. The Zcvalue at each point and the stage assignment are shown in Table 1, and Fig. 2 shows X-ray diffraction patterns of pure stage 2-6 GICs. These results show that the anodic peak “B” in Fig. I(a) corresponds to the formation of the stage 2 GIC from the stage 3, and that another peak “A” is the envelope of peaks due to the conversions from stage m + 1 to stage m (where m 1 3). The current increase “c” does not represent the stage transition but the oxidation of the solution. It is noteworthy that the interplanar distance d, of the FeCl; intercalated compounds, namely 9.37 A, is close to that of the FeCl,-GIC (9.38 A) obtained by the vapour phase method[ lo- 1 l] and those of the related ternary compounds (9.21-9.40 A) obtained from organic solutions[4,6]. During the reverse potential sweep, no peak was observed in the cyclic voltammogram. This is explained on the basis of the reaction on the counter electrode. When the FeCl; ion is intercalated into graphite, the following reaction occurs on the counter electrode: H30+ + e- -
MH2 + H20.
However, this electrochemical reaction could not be reversed during the reverse potential sweep, because there is no source of hydrogen. This prevents de-intercalation of FeCl; on the working electrode. In fact, even after a reverse potential sweep to 800 mV [point
Fig. 1. Typical cyclic vohammogram obtained in aqueous FeQ-HCl solution at a sweep rate of 0.1 mV/s for (a) HOPG during the intial potential sweep; (b) nongraphitized ex-PAN carbon fiber without HOPG; (c) HOPG during the nth potential sweep.
window (i.e., the electrochemical oxidation should be performed below the potential of evolution of oxygen). This difficulty can be partly overcome by controlling the acidity of the solution. Because of the large amount of HCl in the solution used in this work,
the potential ofthe evolution of oxygen rises to about 1300 mV. Another advantage accruing from the use
Table I. Staging of FeCl;-graphite intercalation compounds; the identification of each point is explained in the text and in Fig. I(a) I,. value (A)
Point
: 2 ; :
26.25 (= 22.62 (= 19.29 (= 16.16(= 16.16and 12.71 (= 12.71
950 + 9.21 + 9.34 + 9.46 + 12.71 9.36 +
5(3.35)) 4(3.35)) 3(3.35)) 2(3.35)) 3.35)
Staging of GIC expected by I, graphite and higher stages 6th 5th 4th 3rd 3rd and 2nd 2nd
2nd
225
Electrochemical intercalation of FeCI-,
stage
2
15
;:m 08
stage
8
W
4
8 cu B
1
5
10
( deg.)
z 0
t= 0 0
;: 4
4: 0
il, 10
,_, 15
1
5
Fig. 2. X-ray diffraction (Mokcu) patterns of FeCl;-GIG.
i in Fig. I(a)], the sample shows a stage 2 structure. The voltammogram obtained during the nth potential sweep (n 2 2), which is shown in Fig. 1(c), has no anodic peak. As the starting sample of the nth potential sweep is a stage 2 GIC, further intercalation does not occur and hence no current flows. Figure 3 shows the electrode potential versus quantity of electricity. representing the galvanostatic oxidation process of HOPG in FeC&-HCI-I&O mixture. To determine the assignment of stage structures corresponding to the slopes and plateaus of the curve, oxidation was stopped at several points indicated in Fig. 3 and the X-ray diffraction pattern of the sample was observed. Table 2 summarizes the staging in each sample. The results show that the well-defined plateau “A” and the succeeding slope in Fig. 3 correspond to the conversion from the stage 3 GIC to the stage 2, and that the plateau “lj” is not att~butable to the stage transition but rather to the oxidation of the
solution. Other indications of stage transitions are rather difficult to recognize in the electrode potential versus quantity of electricity curve. The total charge needed to form a stage 2 GIG was found to be 0.082 Ah/g-graphite, and the C/FeCl; ratio of the compound can be estimated on the basis of this value. The ratio obtained is shown in Table 3 together with the ratios for stage 3,4, and 5 GICs. The C/Fe ratios determined by elemental analysis are also shown in Table 3. The C/FeCl; ratio of each GIG is almost equal to the corresponding C/Fe ratio for stage 2 and 3; the small difference found in stage 4 or 5 CIC is probably attributable to experimental error in the quantitative analysis of small amount of iron. This agreement of the C/FeCI, and C/Fe ratios means that all the iron chloride moiecules are intercalated electrochemically and that no electric current is wasted by any side reaction. The ratios can be expressed as n X (- 14), where p?is the stage number. C/Fe ratios of the FeCI,-GIC obtained by the vapour
Table 2. Staging of FeCI; -GIG; the identification of each point is explained in the text and in Fig. 3
Point 5
Fig. 3. Galvanostatic
10 Electric quantity ( ~10~~Ah,~-~ra~it~’ )
oxidation of HOPG FeCI,-HCI solution.
in aqueous
; 6’ c z
Staging of GIG expected from diffraction patterns graphite and higher stages 5th 4th 3rd 2nd 2nd
226
H. SHIOYAMA~~~. Table 3. C/FeC& and C/Fe ratios ofFeC&-GIC
Stage
C/FeCU ratio determined by calculation of the passed charge
C/Fe ratio determined by elemental analysis
2nd
27.2(= 2 x 13.6)
27.7(= 2 x 13.9)
3rd 4th 5th
40.6(= 3 x 13.5) 60.5 (= 4 x 15.1) 13.7 (= 5 x 14.7)
42.9 (= 3 x 14.3) 48.0(= 4 X 12.0) 57.7 (= 5 x 11.5) C
phase reaction are known to be expressed by n X (6 - 7)[ 10,12]. This difference may be accounted for by the existence of co-intercalated molecules such as HCl or H,O. The F&l;-GIC tends to decompose to higher stage compounds in contact with water (e.g., stage 2 GIC decomposes to stage 3 or 4). The decomposition does not take place if the FeCI;-GIC is washed with 6-M HCI, and yet the washed sample decomposes within a few days on exposure to air. This particular decomposition is not observed with the FeCI,-GIC obtained by the vapour phase method. It suggests that the intercalate in the FeCl;-GIC under consideration is distinct from that in FeCI,-GIC even though the d, values are the same. As shown in Fig. 4, the MGssbauer spectrum of stage 2 FeCl;-GIC has a single intense line centered at 0.2 mm/s. This isomer shift of the absorption line of Fe3+ is equal to that of FeCl; crystal. On the other hand, the isomer shift of Fe3 in FeCl,-GIC obtained by the vapour phase method (0.4 mm/s) is comparable to that of FeCl, crystal[ i 31. These findings suggest that the environment of Fe3+ in the Feel;--GE is almost similar to that in the FeCl; crystal. Consequently, they confirm that iron chloride exists as FeCl; anion in FeCl;-GIC.
200
400 T
6 (‘C)
Fig. 5. Thermogravimetry curve for stage 2 FeCI;-GIC Nz stream. Heating rate; 300 deg/h.
in
Figure 5 shows the~o~avimet~ (TG) data relating to stage 2 FeCl;-GIC. The presence of a weight loss in the temperature range below lOo”C, which was not observed for FeCl,-GIC obtained by the vapour phase method[ 141, is taken to be due to the evacuation of co-intercalated species from the interlayer spaces ofgraphite. The second significant weight loss, in the temperature range between. 150 and 4X%, could be due to liberation of iron chloride; the shape of the TG curve in this temperature region is similar to that of FeCl,-GIC[ 141. The existence of co-intercalated species in FeCl;-GIC, predicted by considering the discrepancy between C/Fe ratio of FeC&GIC and that of FeCl,-GIC, is thus proved by TG.
Acknowledgements-This work was partly supported by a grant for International Joint Research from NEDO, Japan. We are also grateful for a grant from Science and Technology Agency, Japan (to H.S.).
REFERENCES
-6
1
#
-4
-2
1
0 Velocity
4
I
4 2 ( mm/s )
6
Fig. 4. Miissbauer spectra of stage 1 FeC13-GIC obtained
from a vapour phase, (CH&N+FeCl; and stage 2 FeCI;GIG.
I. A. H&old, Bull. Sot. Chim. France 999 ( 1955). 2. M. Inagaki and Z. D. Wang, Synth. Met. 20, 1 (1987). 3. M. Inagaki, Z. D. Wang, Y. Okamoto, and M. Ohira, Sy&. Met. 20,9 (1987). 4. M. Inagaki, T. Mitsuhashi, and Y. Soneda, J. Chim. Phyz 84, 1439 ( 1987). R. SchlGgl and H. P. Boehm, Synth. Met. 23, 407 (1988). 6. D. Billaud, A. El Haouari, and R. Gerardin, Synth. Met. 29, F241 (1989). 7. D. Biilaud and A. El Haouari, Synth. Met. 34, 79 ( i989). 8. M. Futamata, Exzd. Abst. Int. Symp. on Carbon. Tsukuba, Japan, p. 8 18(I 990). 9. M. D. Luter and D. L. Wertz, J. Phys. Chem. 853542 (1981). 10. J. G. Hooley and M. Bartlett, Carbon 5,4 17 ( 1967). Il. D. Hohlwein, F. D. Grigutsch, and A. Knappwost, Angew. Chem. (Inr. Ed.] 9.382 ( 1969). 12. J. G. Hooley and R. N. Soniassy, Carbon 8,19 I (1970). 13. R. H. Herber and M. Katada. J. Inora. Nucl. Chem. 41. 1097 (1978). 14. T. Sugiura, M. Sate and K. Fujimoto, Proceed. Jpn. Symp. Carbon, p. 18 ( 1985).
OCOX-6223/93 $6.00 + .@I Copyright 0 1993 Pergamon Press Ltd.
1993
ELECTROCHEMICAL OXIDATION OF GRAPHITE IN AN AQUEOUS MEDIUM: INTERCALATION OF FeCl; H. SHIOYAMA, M. CRESPIN, A. SERON, R. SETTON, D. BONNIN, and F. BEGUIN C.R.M.D., C.N.R.S., I b, rue de la F&ollerie, 4507 I Orl&ans, France (Received 6 May 1992; accepted in revisedform 24 July 1992) Abstract-Graphite was oxidized electrochemically in an aqueous solution of 2.5 moles of FeCl,, I .75 moles of HCI and 6 moles of HzO, which leads to the formation of stage n iron chloride-graphite intercalation compounds (GICs) (where n > 2). The interplanar distance d, of the GIC is -9.4 A; this value is similar to that of the FeClj-GIC obtained by the vapour phase method. Miissbauer spectroscopy shows that the FeC14 ion exists in the interlayer spacing of graphite. The existence of co-intercalated species is suggested by the results of elemental analysis, thermogravimetry and so on. Key Words-Electrochemical
intercalation, water, ferric chloride, hydrochloric acid.
1. INTRODUCHON
2. EXPERIMENTAL
Although Graphite intercalation compounds (GICs) with metal chlorides are usually prepared by the twozone method[ I], the preparation of the GIC from a liquid phase has also been studied by many researchers. Molten metal chloride systems were used successfully to synthesize corresponding GICs[2,3]. Metal chlorides dissolved in organic solvents were also used for GIC syntheses, with the oxidant necessary to drive the reaction provided by gaseous chlorine bubbled through the solution[4], formed by a photochemical reaction[S], or created by an electric current[6,7]. In these cases, solvents were always cointercalated, giving GICs with properties different from those ofthe corresponding binary GIC obtained by vapour phase intercalation. Compared with other preparation methods, the electrochemical preparation of GICs has several advantages. For example, the stage of the sample can be correlated with the applied potential or the charge passed during the electrochemical intercalation, hence this process is applicable not only to the analysis of the formation process of GIC but also to the well-controlled synthesis of GIC. Water has never been used as an electrolytic solvent for the electrochemical intercalation of metal halides because of its limited potential window. In 1990, Futamata[8] gave a clue to the electrochemical intercalation of ZnBr, into carbon plastic electrodes by repeating charge-discharge cycles in aqueous solutions, but intercalation into ordinary graphite using a conventional electrochemical technique was unsuccessful. In this paper, we present the electrochemical preparation of FeCl;-GIC in an aqueous medium. Some properties of the product are also described.
The composition of the solution used was 2.5 moles of FeCl,, 1.75 moles of HCl, and 6 moles of H,O. Highly oriented pyrolytic graphite (HOPG) from Union Carbide was used as the host. A slab of HOPG (ca. 10 mg) was used for the working electrode; electrical contact was made by a bundle of nongraphitized ex-PAN carbon fiber. A glassy carbon rod was used for the counter electrode and the saturated calomel electrode (SCE) was adopted as a reference electrode, to which all potential values will be referred hereafter. Cyclic voltammetry was carried out with the help of a triangle wave signal generator (Tacussel, type GSTP4) and a potentiostat (Tacussel, type PRT 1O0,5). Galvanostatic experiments were performed using a DC current generator. The relation between the potential of the working electrode and electric quantity transferred in the cell was continuously measured with a high impedance X/t-recorder. The electrochemical treatments could be stopped at any desired time to obtain the X-ray diffraction pattern of the sample. During this measurement, the sample was covered with thin polyethylene film (0.02-mm thick) to prevent decomposition by moisture. After washing with a large amount of 6-N HCl solution to remove any adhering iron chloride, the samples were transferred into ampoules, sealed under vacuum, and sent to the Service Central de Microanalyse du CNRS (Vernaison, France) for elemental analyses. MGssbauer spectra were recorded using a Lescint AME Mijssbauer driver with a lo-mCi 57Cosource in rhodium matrix at room temperature. As references (CH,),N+FeCl; crystal and FeCl,-GIC obtained by vapour phase synthesis were used. 3. RESULTS AND DISCUSSION
Please send correspondence concerning this paper to Dr. H. Shioyama, Government Industrial Res. Inst., Osaka, Midorigaoka I-8-31, Ikeda, Osaka 563, Japan.
223
One of the major problems of water as a solvent for electrochemical reaction is its limited potential
224
H.
1 (mA) 1.04
1(mA)
f
(b)
1.0
eta1
SHIOYAMA
I
of such a concentrated solution of hydrochloric acid is that FeCl; is the average and only important solute formed from FeClJ9]. Hence, it was expected that the FeCld ion could be intercalated during the anodic oxidation of graphite in this solution. A typical cyclic voltammogram obtained during the initial potential sweep is shown in Fig. l(a). Compared with a voltammogram [Fig. l(b)] of nongraphitized ex-PAN carbon fiber (i.e., the voltammogram of the working electrode without HOPG), anodic peaks are observed. In order to interpret the voltammogram, the potential sweep was stopped at several points (a to h) indicated in Fig. l(a) to obtain X-ray diffraction patterns ofthe sample. The Zcvalue at each point and the stage assignment are shown in Table 1, and Fig. 2 shows X-ray diffraction patterns of pure stage 2-6 GICs. These results show that the anodic peak “B” in Fig. I(a) corresponds to the formation of the stage 2 GIC from the stage 3, and that another peak “A” is the envelope of peaks due to the conversions from stage m + 1 to stage m (where m 1 3). The current increase “c” does not represent the stage transition but the oxidation of the solution. It is noteworthy that the interplanar distance d, of the FeCl; intercalated compounds, namely 9.37 A, is close to that of the FeCl,-GIC (9.38 A) obtained by the vapour phase method[ lo- 1 l] and those of the related ternary compounds (9.21-9.40 A) obtained from organic solutions[4,6]. During the reverse potential sweep, no peak was observed in the cyclic voltammogram. This is explained on the basis of the reaction on the counter electrode. When the FeCl; ion is intercalated into graphite, the following reaction occurs on the counter electrode: H30+ + e- -
MH2 + H20.
However, this electrochemical reaction could not be reversed during the reverse potential sweep, because there is no source of hydrogen. This prevents de-intercalation of FeCl; on the working electrode. In fact, even after a reverse potential sweep to 800 mV [point
Fig. 1. Typical cyclic vohammogram obtained in aqueous FeQ-HCl solution at a sweep rate of 0.1 mV/s for (a) HOPG during the intial potential sweep; (b) nongraphitized ex-PAN carbon fiber without HOPG; (c) HOPG during the nth potential sweep.
window (i.e., the electrochemical oxidation should be performed below the potential of evolution of oxygen). This difficulty can be partly overcome by controlling the acidity of the solution. Because of the large amount of HCl in the solution used in this work,
the potential ofthe evolution of oxygen rises to about 1300 mV. Another advantage accruing from the use
Table I. Staging of FeCl;-graphite intercalation compounds; the identification of each point is explained in the text and in Fig. I(a) I,. value (A)
Point
: 2 ; :
26.25 (= 22.62 (= 19.29 (= 16.16(= 16.16and 12.71 (= 12.71
950 + 9.21 + 9.34 + 9.46 + 12.71 9.36 +
5(3.35)) 4(3.35)) 3(3.35)) 2(3.35)) 3.35)
Staging of GIC expected by I, graphite and higher stages 6th 5th 4th 3rd 3rd and 2nd 2nd
2nd
225
Electrochemical intercalation of FeCI-,
stage
2
15
;:m 08
stage
8
W
4
8 cu B
1
5
10
( deg.)
z 0
t= 0 0
;: 4
4: 0
il, 10
,_, 15
1
5
Fig. 2. X-ray diffraction (Mokcu) patterns of FeCl;-GIG.
i in Fig. I(a)], the sample shows a stage 2 structure. The voltammogram obtained during the nth potential sweep (n 2 2), which is shown in Fig. 1(c), has no anodic peak. As the starting sample of the nth potential sweep is a stage 2 GIC, further intercalation does not occur and hence no current flows. Figure 3 shows the electrode potential versus quantity of electricity. representing the galvanostatic oxidation process of HOPG in FeC&-HCI-I&O mixture. To determine the assignment of stage structures corresponding to the slopes and plateaus of the curve, oxidation was stopped at several points indicated in Fig. 3 and the X-ray diffraction pattern of the sample was observed. Table 2 summarizes the staging in each sample. The results show that the well-defined plateau “A” and the succeeding slope in Fig. 3 correspond to the conversion from the stage 3 GIC to the stage 2, and that the plateau “lj” is not att~butable to the stage transition but rather to the oxidation of the
solution. Other indications of stage transitions are rather difficult to recognize in the electrode potential versus quantity of electricity curve. The total charge needed to form a stage 2 GIG was found to be 0.082 Ah/g-graphite, and the C/FeCl; ratio of the compound can be estimated on the basis of this value. The ratio obtained is shown in Table 3 together with the ratios for stage 3,4, and 5 GICs. The C/Fe ratios determined by elemental analysis are also shown in Table 3. The C/FeCl; ratio of each GIG is almost equal to the corresponding C/Fe ratio for stage 2 and 3; the small difference found in stage 4 or 5 CIC is probably attributable to experimental error in the quantitative analysis of small amount of iron. This agreement of the C/FeCI, and C/Fe ratios means that all the iron chloride moiecules are intercalated electrochemically and that no electric current is wasted by any side reaction. The ratios can be expressed as n X (- 14), where p?is the stage number. C/Fe ratios of the FeCI,-GIC obtained by the vapour
Table 2. Staging of FeCI; -GIG; the identification of each point is explained in the text and in Fig. 3
Point 5
Fig. 3. Galvanostatic
10 Electric quantity ( ~10~~Ah,~-~ra~it~’ )
oxidation of HOPG FeCI,-HCI solution.
in aqueous
; 6’ c z
Staging of GIG expected from diffraction patterns graphite and higher stages 5th 4th 3rd 2nd 2nd
226
H. SHIOYAMA~~~. Table 3. C/FeC& and C/Fe ratios ofFeC&-GIC
Stage
C/FeCU ratio determined by calculation of the passed charge
C/Fe ratio determined by elemental analysis
2nd
27.2(= 2 x 13.6)
27.7(= 2 x 13.9)
3rd 4th 5th
40.6(= 3 x 13.5) 60.5 (= 4 x 15.1) 13.7 (= 5 x 14.7)
42.9 (= 3 x 14.3) 48.0(= 4 X 12.0) 57.7 (= 5 x 11.5) C
phase reaction are known to be expressed by n X (6 - 7)[ 10,12]. This difference may be accounted for by the existence of co-intercalated molecules such as HCl or H,O. The F&l;-GIC tends to decompose to higher stage compounds in contact with water (e.g., stage 2 GIC decomposes to stage 3 or 4). The decomposition does not take place if the FeCI;-GIC is washed with 6-M HCI, and yet the washed sample decomposes within a few days on exposure to air. This particular decomposition is not observed with the FeCI,-GIC obtained by the vapour phase method. It suggests that the intercalate in the FeCl;-GIC under consideration is distinct from that in FeCI,-GIC even though the d, values are the same. As shown in Fig. 4, the MGssbauer spectrum of stage 2 FeCl;-GIC has a single intense line centered at 0.2 mm/s. This isomer shift of the absorption line of Fe3+ is equal to that of FeCl; crystal. On the other hand, the isomer shift of Fe3 in FeCl,-GIC obtained by the vapour phase method (0.4 mm/s) is comparable to that of FeCl, crystal[ i 31. These findings suggest that the environment of Fe3+ in the Feel;--GE is almost similar to that in the FeCl; crystal. Consequently, they confirm that iron chloride exists as FeCl; anion in FeCl;-GIC.
200
400 T
6 (‘C)
Fig. 5. Thermogravimetry curve for stage 2 FeCI;-GIC Nz stream. Heating rate; 300 deg/h.
in
Figure 5 shows the~o~avimet~ (TG) data relating to stage 2 FeCl;-GIC. The presence of a weight loss in the temperature range below lOo”C, which was not observed for FeCl,-GIC obtained by the vapour phase method[ 141, is taken to be due to the evacuation of co-intercalated species from the interlayer spaces ofgraphite. The second significant weight loss, in the temperature range between. 150 and 4X%, could be due to liberation of iron chloride; the shape of the TG curve in this temperature region is similar to that of FeCl,-GIC[ 141. The existence of co-intercalated species in FeCl;-GIC, predicted by considering the discrepancy between C/Fe ratio of FeC&GIC and that of FeCl,-GIC, is thus proved by TG.
Acknowledgements-This work was partly supported by a grant for International Joint Research from NEDO, Japan. We are also grateful for a grant from Science and Technology Agency, Japan (to H.S.).
REFERENCES
-6
1
#
-4
-2
1
0 Velocity
4
I
4 2 ( mm/s )
6
Fig. 4. Miissbauer spectra of stage 1 FeC13-GIC obtained
from a vapour phase, (CH&N+FeCl; and stage 2 FeCI;GIG.
I. A. H&old, Bull. Sot. Chim. France 999 ( 1955). 2. M. Inagaki and Z. D. Wang, Synth. Met. 20, 1 (1987). 3. M. Inagaki, Z. D. Wang, Y. Okamoto, and M. Ohira, Sy&. Met. 20,9 (1987). 4. M. Inagaki, T. Mitsuhashi, and Y. Soneda, J. Chim. Phyz 84, 1439 ( 1987). R. SchlGgl and H. P. Boehm, Synth. Met. 23, 407 (1988). 6. D. Billaud, A. El Haouari, and R. Gerardin, Synth. Met. 29, F241 (1989). 7. D. Biilaud and A. El Haouari, Synth. Met. 34, 79 ( i989). 8. M. Futamata, Exzd. Abst. Int. Symp. on Carbon. Tsukuba, Japan, p. 8 18(I 990). 9. M. D. Luter and D. L. Wertz, J. Phys. Chem. 853542 (1981). 10. J. G. Hooley and M. Bartlett, Carbon 5,4 17 ( 1967). Il. D. Hohlwein, F. D. Grigutsch, and A. Knappwost, Angew. Chem. (Inr. Ed.] 9.382 ( 1969). 12. J. G. Hooley and R. N. Soniassy, Carbon 8,19 I (1970). 13. R. H. Herber and M. Katada. J. Inora. Nucl. Chem. 41. 1097 (1978). 14. T. Sugiura, M. Sate and K. Fujimoto, Proceed. Jpn. Symp. Carbon, p. 18 ( 1985).